Nowadays, electrical energy is basically converted into mechanical energy via electric motors worldwide. An overseas survey shows that, in USA, Japan, France and Russia, the electrical energy consumed by electric motors accounts more than 60% of the overall industrial electrical energy consumption; and as investigated by related authorities, in the main electric grids in China, the electrical energy consumed by electric motors accounts 60%-68% of the overall industrial electrical energy consumption, which is approximately the same as that in developed countries. Therefore, countries all over the world are developing and popularizing various advanced technologies and devices so as to prompt the economical operation of electric motors. In China, remarkable effects and experiences have been obtained in the developing and adopting of various apparatuses and technologies that prompt the economical operation of electric motors.
With the development of electrical and electronic technologies, microelectronic technologies, control technologies and other technologies such as manufacturing processing, various speed-regulating apparatuses appear, among which, a frequency-changing type speed-regulating apparatus has the best performance and developing prospect. Particularly, with the application of vector-control technologies and direct torque control technologies, frequency-changing technologies become mature and take a leading position in AC driving due to its broad speed-regulating range, high speed-stabilization precision, rapid dynamic response and its performance of reversible operation in the four quadrants of a rectangular coordinate system. The speed-regulating performance of the frequency-changing technologies is comparable to DC driving. There is a trend that DC driving will be replaced by AC driving.
At present, globally well-known enterprises, including Siemens Electrical Drives Ltd. of Germany, Vaasa Control System Co., Ltd. of Finland, ABB of Switzerland, Schneider Electric Co. of France and Yaskawa of Japan, etc., are dominating the manufacturing of those commercialized large-scale electrical and electronic devices and frequency-changing devices. Products of the above industrially developed countries are also found in related application fields in China. In applying these products, however, one frequency inverter is employed for one electrical function, and one inverter bridge has to be provided for one frequency inverter. If an energy feedback function is required for the frequency-changing type speed-regulating system, another inverter bridge has to be added. Apparently, above configuration not only makes the system bulky and complex with poor stability, but also deteriorate the maintainability and performance-price ratio.
In view of above defects in prior art frequency-changing technologies, the inventor has conducted a large amount of experiments in cranes, oil pumping units used in oil fields, water injection pumps and steel ball machines, etc., on how to realize rotor frequency-changing type speed-regulating simultaneously by driving multiple electric motors asynchronously via an inverter bridge. In addition, a cabinet body and a tank body are designed and manufactured for AC and DC devices, thereby batch production and application are implemented, and a good effect is obtained. In this regard, three Chinese patents have been applied and authorized successively, with patent numbers ZL 200810094147.6, ZL 200810048732.2 and ZL 200810048252.2, respectively.
However, in an electric driving system, on one hand, there exists a power balance equation of electric motors:P1=3U1I1 cos φ1
Wherein    P1—electric motor input power (KW);    U1—stator winding phase voltage (KV);    I1—stator winding phase current (A);    φ1—angle between the phase voltage and the phase current;    3 U1 I1 COS φ1—electric motor power factor.
On the other hand, for easy analysis, under the premise that no stator copper loss and iron loss of an electric motor and no rotor mechanical loss are considered:P1=P2+PS 
Here, P2—electric motor output power, i.e., load power;
PS—electric motor slip power, i.e., the power which is fed back to an electric grid or an electric motor by an inverter bridge after speed regulating of the electric motor.
Moreover, Ps=SP1;
Therefore,
                              P          ⁢                                          ⁢          1                =                ⁢                              P            ⁢                                                  ⁢            2                    +          Ps                                        =                ⁢                              P            ⁢                                                  ⁢            2                    +                      SP            ⁢                                                  ⁢            1                              
Or,
                              P          ⁢                                          ⁢          2                =                ⁢                              P            ⁢                                                  ⁢            1                    -                      SP            ⁢                                                  ⁢            1                                                  =                ⁢                  P          ⁢                                          ⁢          1          ⁢                      (                          1              -              S                        )                              
Based on that in an electric driving system, the slip ratio S of an electric motor is:
  S  =                              N          0                -        N                    N        0              =          1      -              N                  N          0                    
Therefore,
                              P          2                =                ⁢                              P            1                    ⁡                      (                          1              -              S                        )                                                  =                ⁢                              P            1                    ⁡                      (                          1              -                              (                                  1                  -                                      N                                          N                      0                                                                      )                                      )                                                  =                ⁢                              P            1                    ⁢                      N                          N              0                                          
That is,
                              P          1                =                ⁢                                            N              0                        N                    ⁢                      P            2                                                  =                ⁢                              N            0                    ⁢                                    P              2                        N                              
Wherein,    No—rotor synchronous rotating speed of an electric motor;    N—rotor instant rotating speed of an electric motor.
The rotor synchronous rotating speed of an electric motor No is a constant, therefore, it is seen from the above equation that: electric motor output power P1 relates simultaneously to electric motor output power P2 (i.e., load) and electric motor rotor instant rotating speed N.
In conclusion, it is clear that: an electric driving system has three working states: in the first working state, the rotor rotating speed of the electric motor keeps constant, while the system load changes instantaneously; in the second working state, the system load keeps constant, while the rotor rotating speed of the electric motor changes instantaneously; and in the third working state, both the system load and the rotor rotating speed of the electric motor change instantaneously.
The prior art designs of an electric motor fail to satisfy the requirements of an electric driving system, in other words, an electric motor can not operate constantly in a high-efficiency region by these designs. Furthermore, during the practical operation of an electric driving system, technicians only contribute to an electric driving system of above discussed first or the second working state. Electrical driving systems of the first and the second working states have been discussed in Practical Manual For Energy-Saving Reforming On An Electric Motor (published by Shanghai Science Publishing House) and related patent documents. However, with the rapid development of science and technology and the urgent need of energy saving, an electrical driving and controlling system applicable for above discussed third working state is desired.